1. Field of Invention
The present invention relates to an improved clarifying agent composition comprising an aditol diacetal and 0.05 to 50 parts by weight of an organosilane treated fume silica having a pH value of 5.5 to 8, measured in a 4% w/w dispersion in 1:1 mixture of water-methanol.
This invention also intents to provide a new manufacturing method for preparing superfine powdery diacetal composition with high dispersivity.
Finally, the present invention also relates to use of said compositions for preparing polyolefin plastic article without the visible white spots on the surface of the corresponding article.
2. Description of the Related Art
Diacetal powders can be added to polymeric materials, such as polyolefin, as a nucleating agent for facilitating crystallization, reducing forming period during melting and improving physical properties of polyolefin. Diacetal powders can also be used as clarifying agents to increase transparency of semi-crystalline polymers.
Basically, the mechanism of diacetal powders as clarifying agents is described hereafter. At first, diacetal powders are added to polyolefin and melted at a proper treatment temperature. After polyolefin have cooled down, diacetals crystallize to form a crystallizing network and many spherical nucleation sites in polyolefin. Because the nucleation sites are too small to refract light, the polyolefin thus become transparent. Therefore, keeping small particle size to prevent aggregation of diacetal is known to serve as an important factor to improve polyolefin optical properties.
When the particle size of a diacetal decreases, the amount of pulverizing energy required increases, which generally leads to reduction in productivity and rise in production cost. In addition, it has been pointed out that even when the size of the diacetal is reduced while the pulverizing capability is improved, the fluidity is reduced due to an increase in friction and aggregation of the diacetal particles and an increase in the ratio of water adhering to the surface of the diacetal particles under high humidity conditions.
When particle size of the diacetal powders added to polyolefin are too large or aggregated intensively, the polyolefin often show visible white spots. These white spots may lead to rejection (or disqualification) of a finished plastic article. Therefore, removal of the visible white spots in polyolefin is an important issue in the art.
U.S. Pat. No. 5,198,484 Patent (Mannion) described a conventional method for removing the white spots in polyolefin by adding polar aliphatic additives to polyolefin for dispersing diacetal powders. However, the polar aliphatic additives may migrate to the surface of the polyolefin which causes blooming. Another conventional method is to raise the treatment temperature 3 to 10° C. higher than the melting point of the diacetals. However, the melting points of the diacetals are usually 50˜100° C. higher than the melting points of the polyolefin, causing the polyolefin to degrade and may form yellowed and olfactory products.
U.S. Pat. No. 4,954,291 Patent (Kabayashi et al.) disclosed a mixed diacetal composition comprising acetal and triacetal by-products and having a lower melting point. Unfortunately, the mixed diacetal composition has limited use as a clarifying agent.
Another conventional method for manufacturing diacetal disclosed in Taiwan Patent No. 565562 (Scrivens et al.) also incurs problems with acetal by-product and triacetal by-product which is difficult to remove from the desired diacetal. This method is also complicated and offers limited control on the particle size of the diacetal which makes the dispersion of diacetal in the polyolefin difficult.
There is a common industrial approach involving pre-blending a powdery compound and a flowing aid to improve the flow properties. Although various types of such fine powder of an inorganic compound are applicable, the use of fine powder of silicone dioxide (silica) has been generally suggested.
U.S. Pat. Application No. 2007-0060697 (Li et al.) described the use of pre-blending a commercial sorbitol acetal compound and a hydrophobic silica for improving the flow properties. Li et al. selected some commercial grades samples of silica from Aerosil® product line “R” series and Sipernat® product lines “D” series from Degussa AG and Cab-o-Sil® product line “TG” series and Nanogel® product line from Cabot. Include Aerosil® R972 and HDK® H15.
Commercially, a hydrophobic treatment of silica has been performed through the use of volatile silanes in a reactor heated at about 400° C. For example, a method to utilize the thermal decomposition oxidizing reaction in an oxyhydrogen flame of silicon tetrachloride gas has been used, wherein the following reaction occurs:SiCl4+H2+O2→SiO2+4HCl
Because it is not very easy to remove hydrogen chloride generated during this reaction, the resulting pH value of the hydrophobic silica thus decreases to 3 to 4. Specifically, the conventional organosilane treated fume silica obtained in the past had various problems such as the pH value and the amount to be added.
Li et al. disclosed a method involving pre-blending an acetal and a hydrophobic silica having low pH value. However, Li et al. did not mention that acidic condition of the hydrophobic silica probably causes the said acetal decay at the polymer processing temperature. Furthermore, it is easy to get the information from the supplier web site that hydrophobic silica Aeosil® R972 has a pH value of 4.2 and HDK® H15 has a pH value of 3.8˜4.8, measured in a 4% w/w dispersion in 1:1 mixture of water-methanol.
Hence, there is a need to provide a simple, low-cost method for preparing clarifying agent composition with small particle size and non-aggregation property that will not decay at the polymer processing temperature. This invention provides an improved superfine powdery clarifying agent composition and manufacturing method thereof to mitigate or obviate the aforementioned problems and is further described below.